1. Field of the Invention
The present invention relates to magnetic discs for use in computer disc drives, and, more particularly, to application of the lubricant layer over the magnetic disc.
2. Description of the Related Art
Computer disc drives commonly use components made out of thin films to store information. Both the read-write element and the magnetic storage media of disc drives are typically made from thin films.
FIG. 1A is an illustration showing the layers of a conventional magnetic media structure including a substrate 105, a seed layer 109, a magnetic layer 113, a diamond like carbon (DLC) protective layer 117, and a lube layer 121. The initial layer of the media structure is the substrate 105, which is typically made of nickel-phosphorous plated aluminum or glass that has been textured. The seed layer 109, typically made of chromium, is a thin film that is deposited onto the substrate 105 creating an interface of intermixed substrate 105 layer molecules and seed layer 109 molecules between the two. The magnetic layer 113, typically made of a magnetic alloy containing cobalt (Co), platinum (Pt) and chromium (Cr), is a thin film deposited on top of the seed layer 109 creating a second interface of intermixed seed layer 109 molecules and magnetic layer 113 molecules between the two. The DLC protective layer 117, typically made of carbon and hydrogen, is a thin film that is deposited on top of the magnetic layer 113 creating a third interface of intermixed magnetic layer 113 molecules and DLC protective layer 117 molecules between the two. Finally the lube layer 121, which is a lubricant typically made of a polymer containing carbon (C) and fluorine (F) and oxygen (O), is deposited on top of the DLC protective layer 117 creating a fourth interface of intermixed DLC protective layer 117 molecules and lube layer 121 molecules.
The durability and reliability of recording media is achieved primarily by the application of the DLC protective layer 117 and the lube layer 121. The combination of the DLC protective layer 117 and lube layer 121 is referred to as a protective overcoat. The DLC protective layer 117 is typically an amorphous film called diamond like carbon (DLC), which contains carbon and hydrogen and exhibits properties between those of graphite and diamond. Thin layers of DLC are deposited on disks using conventional thin film deposition techniques such as ion beam deposition (IBD), plasma enhanced chemical vapor deposition (PECVD), magnetron sputtering, radio frequency sputtering or chemical vapor deposition (CVD). During the deposition process, adjusting sputtering gas mixtures of argon and hydrogen varies the concentrations of hydrogen found in the DLC. Since typical thicknesses of DLC protective layer 117, are less than 100 Angstroms, lube layer 121 is deposited on top of the DLC protective layer 117, for added protection, lubrication and enhanced disk drive reliability. Lube layer 121 further reduces wear of the disc due to contact with the magnetic head assembly.
A typical lubricant used in lube layer 121 is Perfluoropolyethers (PFPEs), which are long chain polymers composed of repeat units of small perfluorinated aliphatic oxides such as perfluoroethylene oxide or perfluoropropylene oxide. As is well known in the art, PFPEs are used as lubricants because they provide excellent lubricity, wide liquid-phase temperature range, low vapor pressure, small temperature dependency of viscosity, high thermal stability, and low chemical reactivity. PFPEs also exhibit low surface tension, resistance to oxidation at high temperature, low toxicity, and moderately high solubility for oxygen. Several different PFPE polymers are available commercially, such as Fomblin Z (random copolymer of CF2CF2O and CF2O units) and Y (random copolymer of CF(CF3)CF2O and CF2O) including Z-DOL and AM 2001 from Montedison, Demnum (a homopolymer of CF2CF2CF2O) from Daikin, and Krytox (homopolymer of CF(CF3)CF2O).
Lube layer 121 is typically applied evenly over the disc, as a thin film, by dipping the discs in a bath containing mixture of a few percent of PFPE in a solvent and gradually draining the mixture from the bath at a controlled rate. The solvent remaining on the disc evaporates and leaves behind a layer of lubricant less than 100 Angstroms. Recent advances have enabled the application of PFPE using an in-situ vapor deposition process that includes heating the PFPE with a heater in a vacuum lube process chamber. In this system, evaporation occurs in vacuum onto freshly deposited DLC protective layer 117 that has not been exposed to atmosphere, creating a thin uniform coating of PFPE lube layer 121.
Since it is known in the art that recording media with higher lubricant bonded ratio has better corrosion protection and that an in-situ vapor lubrication process enhances the bonding between lubricants and amorphous carbon, in-situ vapor lubrication has been used to lubricate amorphous carbon layers. In-situ vapor lubrication of recording media is the lubrication of the recording media immediately after the DLC protective layer 117 has been deposited over the magnetic layer 113 without exposing it to atmosphere.
FIG. 1B is a flow chart showing the typical steps used in an in-situ vapor lubrication process that deposits PFPE lubricant over a carbon layer. The process begins with step 150 by transferring a partially complete media with substrate 105, seed layer 109, and magnetic layer 113 into a vacuum chamber. The transferring process typically involves moving a disk, after depositing a magnetic layer on it, into a carbon deposition chamber without taking it out of vacuum. In step 155 an amorphous carbon layer is deposited over the partially complete media. Typically the amorphous carbon layer is diamond like carbon (DLC) that has been deposited by conventional sputter deposition techniques. Next in step 160, the amorphous carbon is coated with a lube layer 121 of PFPE using an in-situ vapor lubrication process. Finally, in step 165 the lubed magnetic media is transferred to the next manufacturing operation.
The same technology, however, works less effectively with a DLC protective layer 117. When a DLC protective layer 117 is applied over the magnetic layer 115, unpaired carbon electrons pair with hydrogen electrons and dangling carbon bonds are tied up, as illustrated in FIG. 1C. FIG. 1C is an illustration showing the carbon bonds that are not tied up by other carbon atoms being tied up at the surface with hydrogen bonds. The termination of the carbon bonds on the surface by hydrogen effectively reduces the reactive sites. As a result, the bonding sites for lubricant molecules are reduced and therefore the lubricant bonded ratio decreases. This effect is particularly strong when lubricant is deposited in-situ after depositing the DLC protective layer 117, as manifested by the poor adhesion of lube layer 121 to the DLC protective layer 117. Because of this effect, IBD or PECVD processes, which produce DLC protective layer 117, and in-situ vapor lubrication processes, which enhances bonding, have not been combined to achieve the maximum performance.
The conflicting tribological requirements in the data zone (DZ) of a magnetic disc where information is stored and the landing zone (LZ) where a head takes-off and lands often require different lube designs in different zones. For example, bonded lube is more desirable in the DZ where flyability corrosion protection are the primary concerns, whereas sufficient mobile lube is essential in the LZ where wear durability is of greater importance. While the benefit of zone lubrication to satisfy both requirements has been recognized in the art, the known methods generally focus on post-lubrication treatments by either partial removal or by zone radiation. These additional steps could add considerable complexity to the disc manufacturing process. Particularly, in the case of in-situ vapor lubrication process, these post-lubrication treatments defeat the main benefit of the in-situ vapor lube process, i.e., simplicity and low cost.
Therefore what is needed is a system and method which overcomes these problems and makes it possible to apply a lubricant to a carbon overcoat using an in-situ vapor lubrication process that results in a reliable final overcoat with desirable properties. Desirable properties include a resulting lubricant that is bonded to the carbon overcoat more strongly at the data zone than at the landing zone.